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# How to calculate if a dot lies on a line

Posted on 2006-04-11

What is the formula to find out if a third dot lies between two points?

I have (x,y) coordinates for positions A and B. I want to know if postion C is on

the same line that a and b are on.

I thought to use the formula y = mx + b to find the generic formula for the line for the known points A and B. After that plug in the x,y points for C in the line, and

if the line is still true, then line C is in part of the line. But, I do not know how

to do this mathematically. Any suggestions.

y

| .

| . B

| C

| .

|___A__________________x